This invention relates to a system for testing the parameters of drilling mud, and more particularly, to a unit for preparing the drilling mud to be tested.
In the drilling of wells, such as oil or gas wells, by the rotary method, a drilling mud is circulated from the surface of the earth to the drill bit and back to the surface again for the purposes of cooling the drill bit, removing earth cuttings from the bore hole, and imposing a hydrostatic pressure on the drilled earth formations to prevent flow of fluid therefrom into the well bore hole. In a drilling mud containing water and clay, the rheological properties of plastic viscosity, gel strength, and yield point, which must be maintained within limits in order that the drilling fluid remain pumpable and perform its desired functions, depend largely upon the concentration of clay solids and the extent to which the clay solids are hydrated by and dispersed within the water contained in the fluid.
Drilling muds are used under a wide variety of conditions which require that different compositions be used. For example, where the well bore hole passes through formations containing clay, the clay admixes with the drilling mud and this clay is hydrated by and dispersed by the water in the drilling fluid, thereby increasing the concentration of dispersed clay solids. The increase in the concentration of dispersed clay solids deleteriously affects the rheological properties of the drilling mud. Accordingly, where control of rheological properties is important, the drilling mud should have a minimum change in such properties with increasing concentrations of clay solids.
Usually, drilling muds are shear thinning, i.e., they increase in viscosity at low shear rates and decrease in viscosity at high shear rates, whereby the cuttings may be readily separated from the drilling fluid at the surface of the earth and, in the event circulation of drilling fluid is stopped for any reason, the cuttings will be properly suspended by the drilling fluid within the well and not sink to the bottom thereof with resultant danger of sticking drill pipe. The rheological properites of a drilling mud are ordinarily imparted thereto by virtue of employing a clay such as bentonite as one of the constituents. Since one of the functions of a drilling mud is to impose a hydrostatic pressure on the formations penetrated by the well, it is desirable that the drilling mud have the appropriate density, and density of a drilling mud is increased by adding a weighting agent such as barite. Drilling muds also often contain caustic soda which is added to solubilize certain constituents, inhibit fermentation of organic additives, reduce the effect of contaminants picked up during drilling and to affect other results depending on the type of drilling mud being employed.
Another property desired in a drilling mud is that of resisting gelation at high temperature. With increasing depth of the well, the bottom hole temperature increases. In many wells, these temperatures exceed 300.degree. F. With aqueous drilling muds, high temperatures induce cementation reactions between clay minerals and various drilling mud additives. As a result, the drilling mud tends to attain excessively high gel strengths. With gelation, excessively high pump pressures are required to break circulation with the result that often loss of the drilling mud occurs by being forced into permeable formations. Additionally, gelation of the drilling mud can prevent logging tools from reaching the bottom of the well.
Frequently, during the drilling of a well, drilling conditions change. Changes in temperature occur. The character of the formations being drilled may change, as for example, salt may be encountered. Each change in drilling conditions can affect the properties of the drilling mud. Frequently, to counteract the effect of the changed drilling conditions on the properties of the drilling mud, a change in the composition or character of the drilling mud is required.
The foregoing and other considerations dictate that drilling muds be tested under conditions which closely approximate conditions which would be encountered during drilling. By adding different additives, and by subjecting the drilling mud to various conditions of temperature and pressure, a determination can be made as to whether the mud will perform adequately under actual drilling conditions.
Prior art systems for testing drilling muds have the disadvantage of occasional spillage of drilling mud with its attendant inconvenience. The prior art drilling mud testing system, on which the present invention is an improvement, has an accumulator which pressurizes the drilling mud and valves that prevent leakage of pressurized mud into the laboratory where the testing system is operated. If these valves are not properly operated, pressurized drilling mud escapes into the laboratory. Further, in the prior art system drilling mud must be manually transported from the vessel in which it is mixed to the system where it is tested. This also provokes mud spillage. The mud accumulator in this prior system also has the disadvantage that the piston seals are exposed to the mud and subject to increased wear because of this exposure. Furthermore, the seals are not readily accessible for cleaning and/or replacement.
It is an object of the present invention to provide a drilling mud testing system with improved protection against mud spillage.
It is another object of the present invention to provide a drilling mud testing system having an accumulator with improved access to the piston for inspection and routine maintenance.